INVERTEBRATA. 379 



is essentially the structure and action of a nematocyst. The 

 cnidoblast which encloses it comes, when fully developed, to 

 the surface of the ectoderm, and from its free surface 

 there projects a fine stiff protoplasmic process, like one 

 of the sensory hairs of a sensory epithelium, called the 

 cnidocil. It would seem that when any external body 

 touches this process, the protoplasm of the cnidoblast is 

 stimulated to move in such a way as to press on the nemato- 

 cyst, and cause the ejection of the thread (fig. 195, B, c). 

 Apparently some poisonous substance is ejected with it, and 

 it is this which stuns the water-flea that touches a tentacle, 



Fig. 195. STRUCTURE OF HYDRA. 



A, transverse section ; B, small portion of the same enlarged ectoderm to the 

 right, endoderm left. C, an everted nematocyst. (B and after Parker.) 



nematocysts being specially abundant on the tentacles, 

 where they are grouped in " batteries;" New cnidoblasts 

 are continually being formed to replace those which have 

 been "fired." Thus the soft-bodied Hydra, incapable of 

 flight and seemingly quite defenceless, is not only efficiently 

 protected against enemies, but is even enabled to prey upon 

 animals much more active and more highly organized than 

 itself. 



5. Nervous System. If the description given above of 

 the action of a cnidoblast is correct, we have in it an 

 example of a cell which combines the characters of a 

 sensory and of a motor (muscular) cell. A stimulus of a 



